The present invention relates generally to induction motor rotors and to methods for their fabrication.
High power motor applications are generally in the ten kilowatt to one hundred kilowatt range. High speed motor applications are generally in the ten thousand rpm to seventy-five thousand rpm range. Many permanent magnet rotors that have high efficiency in the range of about ninety percent to about ninety-five percent at high power and speed specifications are very expensive to fabricate due to the expense of high energy permanent magnets and the retainers which keep the magnets attached to the rotor. These motors are typically in the fifteen dollar to two hundred dollar per 746 Watts (1 horsepower) range. Many solid rotor induction machines that economically meet high power and speed specifications have very low efficiency due to rotor stray load losses and chopping losses. The use of a laminated core instead of a solid core reduces these losses to a large extent but does not provide the efficiency of permanent magnet motors.
It is therefore seen to be desirable to provide a high speed motor having high efficiency at a reasonable cost. It would additionally be desirable to provide a self-supporting high speed motor rotor without a mechanical retainer in order to achieve low losses at a low cost.
Briefly, according to one embodiment of the present invention, an induction motor rotor comprises a rotor core, which may be solid or may include a plurality of rotor laminations, having rotor bar slots, a plurality of rotor bars positioned in the rotor bar slots, and two rotor end rings brazed to the rotor bars, the rotor bars and rotor end rings pre-stressing the rotor core.